The most complicated stage, known as cavitation, occurs next. The pressure generated in this stage tends to be lower than in the shock phase. During this phase, the stagnation pressure generated from the projectile slows it down in the fluid, transferring its energy to the fluid. Depending on the scenario, this phase may exhibit the highest pressure. The effect of this shock is dispersive and encountered by most of the structure in its path and cone of the wave. The structure and fluid will be impulsively-loaded upon impact, creating a hemispherical shock wave in the fluid that travels at the speed of sound. The shock phase describes the introduction of the projectile into the medium. The phases are shock, drag, cavitation, and exit. HRAM occurs in four distant phases, with each phase accompanied by its own pressure distribution. The nature of this damage will vary based on the nature of the material (e.g., whether it is metallic or composite). There is also damage at the projectile entry and sometimes exit locations. The structural damage can range from complete structural failure to damage of critical internal components. The pressure is very high near the projectile, on the order of 10 ksi, and decreases exponentially away from the projectile but may still carry enough energy to cause catastrophic damage. The projectile will, in turn, transfer its energy to the fluid, creating a very high pressure, which is then imparted onto the structure. A classic case, and common occurrence, is when an aircraft with fuel in the wings is impacted with a fast-moving projectile, which could be foreign object debris or a ballistic projectile. Hydrodynamic ram (HRAM) occurs when a fluid-filled enclosure is penetrated with a high-velocity projectile.
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